Tn9 is a transposible genetic element derived from a Resistance Transfer Factor (RTF) which can be transposed to a number of unrelated genomes including bacteriophage lambda. Tn9 consists of a sequence encoding chloramphenicol acetyl transferase (cam) bracketed by direct repeats of the insertion sequence IS1 (IS1-cam-IS1). We previously isolated a lambda bio (biotin transducing) Tn9 which has the structure lambda bio IS1camIS lambda. In this phage, the IS1 copies bracketing cam can also be considered as bracketing the lamba bio. Our experiments show that this allows the lambda bio to act as a transposon. First, the lambda Tn9 phage was found to lysogenize E. coli under conditions where host recA and phage int xis and red functions were deficient. The frequency of lamba bio lysogens (approximately 10 to the minus 7 per infected cell) was similar to that of transposition of cam. The lambda bio lysogens do not carry cam. The lambda bio could then transpose to at least two sites on an F'gal plasmid.